1. Field
This application relates to electromagnetic coils used in electric generators, electric motors, and other electrical equipment.
2. Prior Art
Most electric generators and electric motors use electromagnetic coils to either convert mechanical motion into electricity (generator) or convert electricity into mechanical motion (motor). These electromagnetic coils are generally made of electrically conductive and insulated wire either wound into a winding without a metal core (coreless) or wound into a winding around a ferromagnetic material (core) such as iron or steel. These coils consist of a single wire wound in many approximately parallel loops (turns) that are flat and unfolded.
It is well known in the art that an electrical current passing through a wire induces a magnetic field that uniformly circles the wire in a plane perpendicular to the direction of current. However, when many wire loops are formed into a coil (winding), current flow creates a magnetic field from each wire that extends to pass through the coil center, concentrating the magnetic field along the coil centerline. The concentrated magnetic field at the coil center can be many times more than the magnetic field elsewhere around the coil.
However, the coreless electromagnetic coil used in some rotating generators or motors cannot take advantage of the concentrated magnetic field as described above. This is simply due to the circular shape of the coil blocks a radial support of the magnet, or the magnet itself, passing through the coil center, or the converse of the coil passing around a magnet on its centerline. Linear generators and motors are an exception because the magnet can have linear support along the coil centerline without interfering with linear motion of either the magnet or coil.
Therefore, coreless electromagnetic coils loose the benefit of the concentrated magnetic field at the coil center to avoid the rotating part (rotor) of a generator or motor colliding with the fixed part (stator) of the generator or motor. For example, a fixed arrangement of coreless coils (stator) can be positioned to one or both sides of magnets on a rotor such that the rotor's rotation causes the magnet centers to momentarily align with the coil centers but the magnets do not pass through the coil centers.
To keep cost and complexity low, coreless electromagnetic coils are often found in simple wind turbines where the wind forces blades to rotate magnets across fixed electromagnetic coils (as described above), generating electricity. Without ferromagnetic cores, the generator is simpler and less expensive, but the power produced is much lower than generators that have electromagnetic coils with ferromagnetic cores.
Coils with ferromagnetic cores take advantage of the concentrated field at coil center, but at a price of more complexity and expense. The coil wire is wrapped around a ferromagnetic core forming an electromagnet with a north or south-pole at the ends, depending on the direction of the current through the coil. If the current direction is reversed, the magnetic poles reverse.
The literature describes many types of generators and motors using electromagnetic coils with cores. In general, the core of electromagnetic coil is positioned opposite a magnet such that a relative movement induces a magnetic field in the core. That relative movement can be either coils rotating about fixed magnets, or magnets rotating about fixed coils. The magnets can be permanent, electromagnet, or an electrically conducting cage as in an inductive motor. Furthermore, a circular pattern of coils can be fixed or rotate on the inside or outside of a circular pattern of fixed or rotating magnets. Whatever the arrangement, there are advantages and disadvantages of using ferromagnetic cores in the electromagnetic coils of generators and motors.
The primary advantage is that the core enables access to the concentrated magnetic field in the coil center such that a relative motion of the core and magnet generates more current than a coreless coil.
The disadvantages are more complexity and expense than a coreless coil. In addition, to the added cost of the cores, the introduction of ferromagnetic cores cause a problem called torque cogging, or just cogging. The magnet and core inherently attract each other, and considerable force must be expended to separate them or the rotor will not rotate. This is called cogging, and it is a major problem for generators. For example, considerable wind energy is lost to a wind turbine before the wind is strong enough to overcome cogging and self-start the generator. Cogging also causes instability, vibration, noise, and damage to generators. Since cogging is such a problem, considerable design and operational tradeoffs from optimum performance are made to reduce it. These tradeoffs generally reduce power output, increase cost, and add complexity.
Furthermore, induction generators use electromagnetic coils with cores to control the rotation of the rotor such that a force causing rotation to exceed a prescribed speed generates electrical power. Induction generators have an advantage of generating grid ready power, even when the external force varies. However, to produce useful power, this range of variation can be narrow, forfeiting energy outside the range. Another disadvantage is that the electronics is more complex, and some of the generated power is consumed by the generator itself, in order to keep the coils charged.
Therefore, prior art generators and motors have significant disadvantages.